Biaxially oriented polypropylene (BOPP) films used for packaging, decorative, and label applications often perform multiple functions. For example, in laminations they can provide printability, transparent or matte appearance, and/or slip properties. They can further be used to provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties. They can also be used to provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
However, in recent years, interest in “greener” packaging has been developing. Packaging materials based on biologically derived polymers are increasing due to concerns with renewable resources, raw materials, and greenhouse gases. Bio-based polymers are believed—once fully scaled-up—to help reduce reliance on petroleum, reduce production of greenhouse gases, and can be biodegradable. The biodegradable aspect is of interest to many snack manufacturers so as to provide litter abatement in addition to a lower carbon footprint package. Bio-based polymers such as polylactic acid (PLA)—which is currently derived from corn starch (but can be derived from other plant sugars) and thus, can be considered to be derived from a renewable or sustainable resource—is one of the more popular and commercially available materials available for packaging film applications. Other bio-based polymers such as polyhydroxyalkanoates (PHA) and particularly, polyhydroxybutyrate (PHB), are also of high interest.
There are several manufacturers of oriented PLA films, in particular biaxially oriented PLA, but none of them provides a satisfactory moisture barrier property when metallized. For example, Celplast Metallized Products, Ltd.'s Enviromet™ high barrier metallized PLA film typical data sheet describes a product that exhibits an oxygen barrier of 6.2 cc/m2/day (at 23° C., 50% relative humidity or RH) but a relatively poor moisture barrier of 3.1 g/m2/day (at 38° C., 90% RH) as compared to typical metallized biaxially oriented polypropylene films.
For such a bio-based polymer to be useful for snack food packaging applications, it is desirable that the bio-based polymer film match as many of the attributes possible that BOPP is well-known for, such as heat sealability, printability, controlled COF, metallizability, barrier, etc. In particular, for high barrier packaging, metallized oriented PLA films should demonstrate good oxygen and moisture barrier properties. For metallized oriented PLA in particular, good oxygen barrier property is generally easily achieved due to the polar nature of PLA, which provides good hydrogen-bonding of the polymer molecules. However, this polar nature tends to be detrimental for achieving high moisture barrier. Without being bound by any theory, the thought is that water molecules—being polar themselves—may more easily migrate through a polar polymer film than a non-polar polymer film.
High barrier metallized BOPP such as Toray Plastics (America), Inc.'s PWX3 product typically demonstrates oxygen barrier of 15.5 cc/m2/day (23° C., 0% RH) and moisture barrier of 0.155 g/m2/day (38° C., 90% RH). Another manufacturer of barrier PLA film, Alcan Packaging Inc., produces a silicon oxide coated PLA film under the tradename Ceramis® whose typical data sheet shows an oxygen barrier of 7.75 cc/m2/day (23° C., 50% RH) and moisture barrier of 7.75 g/m2/day (38° C., 90% RH). Biofilm S. A. promotional literature (such as presented at the “Innovation Takes Root” conference hosted by NatureWorks LLC at Las Vegas, Nev. Sep. 16-18,2008) discusses transparent barrier PLA films demonstrating 3-10 g/m2/day (38° C./90% RH) using various vacuum chamber deposition processes. Vacuum deposition of metal on top of these transparent barrier layers (e.g. silicon oxide, aluminum oxide, or other coatings) to improve moisture barrier properties is costly due to the multiple processing steps required.
U.S. patent application Ser. No. 10/400,056 publication number 2004/0191541 A1 describes a multi-layer metallized film comprising a polyolefin core layer (typically of polypropylene), a polyolefin-based tie-layer on one side of the core layer, and a metal receiving layer of PLA on the tie-layer opposite the core layer side. The PLA layer is metallized. Gas barrier property of this metallized film is very good, with the examples citing moisture barrier as 0.054-0.093 g/m2/day (38° C., 90% RH) and oxygen barrier as 1.09-1.24 cc/m2/day (23° C., 0% RH). However, such a film formulation is primarily based on petrochemicals (such as polypropylene or copolymers thereof), being 96-98 wt % of the total film structure. PLA bio-polymer is only 2-4 wt % of the film. Thus, such a film cannot claim to be a “sustainable” film, being made from a majority amount of petroleum-based resins versus bio-based resins, and also will not be biodegradable.
U.S. Pat. No. 7,128,969 describes a film composed of a base layer of PLA with a minority component of a thermoplastic or polyolefin such as polypropylene or polyethylene, typically less than 1% by weight of the base layer. Such a formulation is particularly suitable for thermoforming or biaxial stretching by means of pneumatic drawing or other mechanical forming. In addition, the use of polyolefin additives such as polypropylene or polyethylene will cause incompatibilities with the polylactic acid polymer resulting in a hazy film appearance. The invention does not contemplate such a structure or formulation for metallizing or for high barrier applications. Nor does it contemplate multi-layer film designs utilizing polyolefin-based metal receiving layers wherein the majority component of such a layer is polyolefin and not PLA.
EP Patent 01385899 describes a multi-layer film design using a PLA base layer formulated with a cyclic polyolefin copolymer (COC) as a cavitating agent to produce an opaque biaxially oriented PLA film. This patent does not contemplate metallization of such a film with superior gas and moisture barrier properties, nor such a film with a polyolefin, non-PLA metal receiving layer.
EP Patent 01385700 describes a biaxially oriented PLA film with good antistatic properties by incorporating antistatic additives such as glycerol monostearate (GMS) into the base layer of PLA. However, the invention does not contemplate such a structure or formulation for metallizing or for high barrier applications. Nor does it contemplate multi-layer film designs utilizing polyolefin-based metal receiving layers wherein the majority component of such a layer is polyolefin and not PLA.
U.S. Pat. No. 7,354,973 describes a polylactic acid composition of 60-97 wt % of PLA and about 3-40 wt % of an ethylene copolymer impact modifier of 20-95 wt % ethylene, 3-70 wt % of an olefin of the formula CH2═C(R1)CO2R2 where R1 is hydrogen or an alkyl group with 1-8 carbon atoms and R2 is an alkyl group with 1-8 carbon atoms, and 0.5-25 wt % of an olefin of the formula CH2═C(R3)CO2R4 where R3 is hydrogen or an alkyl group with 1-6 carbon atoms and R4 is glycidyl. This composition has been found to be suitable as a toughened composition for injection molding applications to prevent brittleness. However, the invention does not contemplate such a structure or formulation for metallizing or for high barrier applications. Nor does it contemplate multi-layer film designs utilizing polyolefin-based metal receiving layers wherein the majority component of such a layer is polyolefin and not PLA.
U.S. Pat. No. 7,368,160 describes biaxially oriented multilayer coextruded polylactic acid films with a PLA skin layer containing 0.05-0.6% of crosslinked polymer antiblock particles. However, the invention does not contemplate such a structure or formulation for metallizing or for high barrier applications. Nor does it contemplate multi-layer film designs utilizing polyolefin-based metal receiving layers wherein the majority component of such a layer is polyolefin and not PLA.
The inventors seek to address the above issues of making high gas and moisture barrier metallized BOPLA films.